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Module title = Tutorial: Basic Electric Stuff
Lesson title = Drawing the ECG!
This is lesson 7 of 7 in this module
In this lesson, we will describe how the vector generates all the waveforms on the ECG tracing. To understand this, you will also need to understand all previous lessons in this module. This lesson will put it all together for you.
In summary, remember that a
depolarization charge moving toward an electrode will create a positive deflection
. In the video that follows, you will see how the vectors create the ECG tracing. We have color coded the ECG to help explain each step of the way. Video has no sound.
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Tissues that are very small, like the SA node and the AV node do not have a strong enough signal to generate a waveform on the ECG. Therefore,
when the SA node fires, it does
not
show up on the ECG
. We indicated this using the
green
color.
In the following image, we can see how the vector for the
atrial depolarization
creates a downward moving electrical charge. The added black circle in the image below represents the positive ECG electrode and in this case, we are using lead II, which would be pointing from about the top left corner of the picture down toward the black circle. Because the atrial waveform is moving toward the positive electrode, the ECG tracing in
yellow
is an upward deflection.
In the following image, atrial depolarization is complete and the electrical activity is inside the AV node.
The AV node is too small to cause a deflection on the ECG
. This is represented in
purple
.
Similar to the AV node, the
purkinje fibers also do not create any deflection
on the ECG tracing (also purple).
This is a very important detail: the ventricular septum is depolarized from the left bundle branch (we drew little tiny branches on it to show that it releases the electrical signal here). Therefore,
the septum depolarizes from left to right
. Look closely at the vector arrow. It is pointing to the left of the image (toward the right ventricle) and therefore, is pointing
AWAY from the positive ECG electrode
. Since the electrical activity at this moment in time is AWAY from the ECG electrode, the
deflection is downgoing
. This is shown in the
blue
tracing.
Very quickly, the septal depolarization is complete and the remaining ventricle depolarizes
from endocardium to epicardium
, resulting in a large electrical signal moving toward the ECG electrode, generating a large upward deflection shown in
pink
.
When this wave of depolarization is completed, the ECG tracing falls back to baseline (orange).
The
ventricles remain depolarized
for a short duration. This results in
no movement of electrical activity
and so we have a
flat segment
on the ECG (green).
When the ventricles begin to repolarize (blue), the
repolarization event is moving away from the ECG electrode
. However, the deflection is UPWARDS! This is important.
Re
polarization is the electrical OPPOSITE to
de
polarization
. Therefore, the deflection that repolarization produces is opposite. Thus, repolarization moving toward the electrode causes a downward deflection, but moving away, causes an
upward deflection in blue
.
Congratulations! This is the end of this module. There were no "practice" sessions in this module, but there will be in the next module as you practice your skills at waveform identification in both normal and abnormal ECGs. This is a difficult skill, so if you have never looked at ECGs before, it might be a little frustrating at first. This is normal.
Lesson 7 of 7
That was the last lesson!
